1. Field of the Invention
The present invention relates generally to the measurement and prediction of scour rate in soils. It has been found that the invention has particular applicability to the measurement and prediction of scour rate in cohesive soils at bridge supports and other structures that obstruct the flow of a body of water.
2. Description of the Related Art
There are approximately 600,000 bridges in the United States, and 500,000 of them are over water. During the last thirty years, over 1,000 of the 600,000 bridges have failed, and 60% of those failures are due to scour of the soil surrounding bridge piers or other supports. Earthquakes, by comparison, account for only 2% of bridge failures. The average cost for flood damage repair of highways on the federal aid system is $50,000,000 per year. Clearly, bridge scour is a significant problem deserving of significant study and attention.
Bridge scour can be divided into general scour, local scour and channel migration. General scour is general erosion of a stream bed without obstacles. Local scour is generated by the presence of obstacles such as piers and abutments, while channel migration is lateral movement of the main stream channel.
When bridges are designed, core samples are usually taken of the soil in the area where the bridge supports will be located. However, these samples are not typically tested to determine their susceptibility to local scour. Rather, a maximum scour depth is calculated and applied to the bridge design regardless of the actual soil present. The scour depth for sand is usually used and, if the soil is more scour resistant than sand, the bridge may be overdesigned, resulting in a significantly higher cost for the structure. If, on the other hand, scour is ignored, the bridge may be prone to failure earlier than planned. It is important, then to be able to accurately predict or forecast the actual rate of scour for a given location as well as the maximum depth of scour that can be expected for a given period of time.
Current scour prediction practice is unable to account for different soil types. Current practice is heavily influenced by two FHWA hydraulic engineering circulars called HEC-18 and HEC-20 (Richarson and Davis, 1995; Lagasse et al., 1995). For pier scour, HEC-18 recommends the use of the following equation to predict the maximum depth of scour ("z.sub.max ") above which all soil resistance must be discounted: EQU z.sub.max =2z.sub.0 K.sub.1 K.sub.2 K.sub.3 K.sub.4 (D/z.sub.0).sup.0.65 F.sub.0.sup.0.43
where z.sub.0 is the depth of flow just upstream of the bridge pier excluding local scour, K.sub.1, K.sub.2, K.sub.3, K.sub.4 are coefficients to take into account the shape of the pier, the angle between the direction of the flow and the direction of the pier, the stream bed topography, and the armoring effect. D is the pier diameter, and F.sub.0 is the Froude number defined as v/(gz.sub.0).sup.0.5 where v is the mean flow velocity and g is the acceleration due to gravity.
However, nothing in HEC-18 gives guidance to calculate the rate of scour in clays and it is implied that the HEC-18 equation should also be used for determining the final depth of scour for bridges on clays. Clays generally scour much more slowly than sand. Thus, using the HEC-18 equation for clays, regardless of the time period over which scour is considered, is probably overly conservative. As a result, bridges constructed based upon such an analysis may be excessively expensive.
In addition, it is probably improper to try to extrapolate a single representative critical shear stress for all clays. Other phenomena, not present in most sands, give cohesion to clays, including water meniscus forces and diagenetic bonds due to aging, such as those developing when a clay turns to rock under pressure and over geologic time. Because of the number and complexity of these phenomena, it is very difficult to predict .tau..sub.c for clays on the basis of a few index properties. As a result, the inventors consider it preferable to measure .tau..sub.c directly for a proposed bridge site.
Some devices are known that have been used to test the scour resistance of cohesive soils. One such device is described by Walter L. Moore and Frank D. Masch, Jr. in "Experiments on the Scour Resistance of Cohesive Sediments," vol. 67, no. 4, Journal of Geophysical Research, pp. 1437-1449 (1962). The device described there is a "rotating cylinder apparatus" wherein a cylinder of cohesive soil 3 inches in diameter and 3 inches long is mounted coaxially inside a slightly larger transparent cylinder that can be rotated at any desired speed up to 2500 rpm. The annular space between the cylindrical soil sample and the rotating cylinder is filled with a fluid to transmit shear from the rotating cylinder to the surface of the soil sample. The soil samples are mounted in the machine with enough water to fill the annular space to the top. The speed of rotation of the outer cylinder is gradually increased until visual observation indicates the presence of scour on the surface of the sample. At this point, a reading is made by a torque indicator. The measured torque is then converted into a shear stress on the soil surface.
There are a number of drawbacks to this type of device. First, the cylindrical soil samples used are mixed to a certain consistency and molded to form the sample. The mixing and molding can materially change the erosion characteristics of the soil being tested since the soil may not be representative of the compaction and consistency of in-place soil.
Further, the method of testing using the rotatable cylinder apparatus requires the sample to be rotated at progressively more rapid rates until erosion or scour is observed. The rate of scour is not tested at a specific velocity and over a specific length of time to provide an erosion rate.
A need exists for devices and methods that can accurately measure and predict scour, scour rates and related information, near bridge piers and the like.